Photochemistry is the branch ofchemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption ofultraviolet (wavelength from 100 to 400 nm),visible (400–750 nm), orinfrared radiation (750–2500 nm).[1]
In nature, photochemistry is of immense importance as it is the basis of photosynthesis, vision, and the formation ofvitamin D with sunlight.[2] It is also responsible for the appearance of DNA mutations leading to skin cancers.[3]
Photochemical reactions proceed differently than temperature-driven reactions. Photochemical paths access high-energy intermediates that cannot be generated thermally, thereby overcoming largeactivation barriers in a short period of time, and allowing reactions otherwise inaccessible by thermal processes. Photochemistry can also be destructive, as illustrated by thephotodegradation of plastics.
When a substance in itsground state (S0) absorbs light, one electron is excited. This electron maintains itsspin. according to the spin selection rule; other transitions would violate the law ofconservation of angular momentum. The excitation to a highersinglet state can be fromHOMO toLUMO or to a higher orbital, so that singlet excitation states S1, S2, S3... at different energies are possible.
Kasha's rule stipulates that higher singlet states quickly relax by radiationless decay orinternal conversion (IC) to S1. Thus, S1 is usually, but not always, the only relevant singlet excited state. This excited state S1 can further relax to S0 by IC, but also by an allowed radiative transition from S1 to S0 that emits a photon; this process is calledfluorescence.
Jablonski diagram. Radiative paths are represented by straight arrows and non-radiative paths by curly lines.
Alternatively, it is possible for the excited state S1 to undergo spin inversion and to generate atriplet excited state T1 having two unpaired electrons with the same spin. This violation of the spin selection rule is possible byintersystem crossing (ISC) of the vibrational and electronic levels of S1 and T1. According toHund's rule of maximum multiplicity, this T1 state would be somewhat more stable than S1.
This triplet state can relax to the ground state S0 by radiationless ISC or by a radiation pathway calledphosphorescence. This process implies a change of electronic spin, which is forbidden by spin selection rules, making phosphorescence (from T1 to S0) much slower than fluorescence (from S1 to S0). Thus, triplet states generally have longer lifetimes than singlet states. These transitions are usually summarized in a state energy diagram orJablonski diagram, the paradigm of molecular photochemistry.
These excited species, either S1 or T1, have a half-empty low-energy orbital, and are consequently moreoxidizing than the ground state. But at the same time, they have an electron in a high-energy orbital, and are thus morereducing. In general, excited species are prone to participate in electron transfer processes.[6]
Photochemical immersion well reactor (750 mL) with a mercury-vapor lamp
Photochemical reactions require a light source that emits wavelengths corresponding to an electronic transition in the reactant. In the early experiments (and in everyday life), sunlight was the light source, although it is polychromatic.[7]Mercury-vapor lamps are more common in the laboratory. Low-pressure mercury-vapor lamps mainly emit at 254 nm. For polychromatic sources, wavelength ranges can be selected using filters. Alternatively, laser beams are usually monochromatic (although two or more wavelengths can be obtained usingnonlinear optics), andLEDs have a relatively narrowband that can be efficiently used, as well as Rayonet lamps, to get approximately monochromatic beams.
Schlenk tube containing slurry of orange crystals of Fe2(CO)9 inacetic acid after its photochemical synthesis from Fe(CO)5. Themercury lamp (connected to white power cords) can be seen on the left, set inside a water-jacketed quartz tube.
The emitted light must reach the targetedfunctional group without being blocked by the reactor, medium, or otherfunctional groups present. For many applications,quartz is used for the reactors as well as to contain the lamp.Pyrex absorbs at wavelengths shorter than 275 nm. Thesolvent is an important experimental parameter. Solvents are potential reactants, and for this reason,chlorinated solvents are avoided because the C–Cl bond can lead tochlorination of the substrate. Strongly-absorbing solvents prevent photons from reaching the substrate.Hydrocarbon solvents absorb only at short wavelengths and are thus preferred for photochemical experiments requiring high-energy photons. Solvents containingunsaturation absorb at longer wavelengths and can usefully filter out short wavelengths. For example,cyclohexane andacetone "cut off" (absorb strongly) at wavelengths shorter than 215 and 330 nm, respectively.
Typically, the wavelength employed to induce a photochemical process is selected based on theabsorption spectrum of the reactive species, most often the absorption maximum. Over the last years[when?], however, it has been demonstrated that, in the majority of bond-forming reactions, the absorption spectrum does not allow selecting the optimum wavelength to achieve the highest reaction yield based on absorptivity. This fundamental mismatch between absorptivity and reactivity has been elucidated with so-calledphotochemical action plots.[8][9]
Continuous-flow photochemistry offers multiple advantages over batch photochemistry. Photochemical reactions are driven by the number of photons that are able to activate molecules causing the desired reaction. The largesurface-area-to-volume ratio of a microreactor maximizes the illumination, and at the same time allows for efficient cooling, which decreases the thermal side products.[10]
Photodegradation of many substances, e.g.polyvinyl chloride andFp. Medicine bottles are often made from darkened glass to protect the drugs from photodegradation.
Alkenes undergo many important reactions that proceed via a photon-induced π to π* transition. The first electronic excited state of an alkene lacks theπ-bond, so that rotation about theC–C bond is rapid and the molecule engages in reactions not observed thermally. These reactions includecis-trans isomerization and cycloaddition to other (ground state) alkene to givecyclobutane derivatives. The cis-trans isomerization of a (poly)alkene is involved inretinal, a component of the machinery ofvision. Thedimerization of alkenes is relevant to the photodamage ofDNA, wherethymine dimers are observed upon illuminating DNA with UV radiation. Such dimers interfere withtranscription. The beneficial effects of sunlight are associated with the photochemically-induced retro-cyclization (decyclization) reaction ofergosterol to givevitamin D. In theDeMayo reaction, an alkene reacts with a 1,3-diketone reacts via itsenol to yield a 1,5-diketone. Still another common photochemical reaction isHoward Zimmerman'sdi-π-methane rearrangement.
In an industrial application, about 100,000 tonnes ofbenzyl chloride are prepared annually by the gas-phase photochemical reaction oftoluene withchlorine.[21] The light is absorbed by chlorine molecules, the low energy of this transition being indicated by the yellowish color of the gas. The photon induceshomolysis of the Cl-Cl bond, and the resulting chlorine radical converts toluene to the benzyl radical:
Coordination complexes andorganometallic compounds are also photoreactive. These reactions can entail cis-trans isomerization. More commonly, photoreactions result in dissociation of ligands, since the photon excites an electron on the metal to an orbital that isantibonding with respect to the ligands. Thus,metal carbonyls that resist thermal substitution undergo decarbonylation upon irradiation with UV light. UV-irradiation of aTHF solution ofmolybdenum hexacarbonyl gives the THF complex, which is synthetically useful:
Select photoreactive coordination complexes can undergooxidation-reduction processes via single electron transfer. This electron transfer can occur within theinner orouter coordination sphere of the metal.[22]
Although bleaching has long been practiced, the first photochemical reaction was described by Trommsdorff in 1834.[23] He observed thatcrystals of the compoundα-santonin when exposed to sunlight turned yellow and burst. In a 2007 study the reaction was described as a succession of three steps taking place within a single crystal.[24]
^CYCLOBUTANE- TYPE PYRIMIDINE DIMERS IN POLYNUCLEOTIDES, R. B. Setlow,Science 1966 Vol. 153, p. 379, DOI: 10.1126/science.153.3734.379
^Klán, Petr; Wirz, Jakob (2009-03-23).Photochemistry of Organic Compounds: From Concepts to Practice. John Wiley & Sons.ISBN978-1-4051-9088-6.
^Turro, Nicholas J.; Ramamurthy, V.; Scaiano, Juan C. (2010).Modern Molecular Photochemistry of Organic Molecules. University Science Books.ISBN978-1-891389-25-2.
^Balzani, Vincenzo; Carassiti, Vittorio (1970).Photochemistry of Coordination Compounds. New York, New York: Academic Press, Inc. pp. 37–39.ISBN978-0-12-077250-6.
^Natarajan, Arunkumar; Tsai, C. K.; Khan, Saeed I.; McCarren, Patrick; Houk, K. N.; Garcia-Garibay, Miguel A. (2007). "The Photoarrangement of α-Santonin is a Single-Crystal-to-Single-Crystal Reaction: A Long Kept Secret in Solid-State Organic Chemistry Revealed".Journal of the American Chemical Society.129 (32):9846–9847.Bibcode:2007JAChS.129.9846N.doi:10.1021/ja073189o.PMID17645337.
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